Where, herein, I employ the words "direct method" I refer to a procedure wherein the major portion of the work is performed within the patient's mouth and by the words "indirect method" I refer to a procedure wherein the major portion of the work is done outside the mouth.
The invention relates to restoration of a mutilated tooth, e.g. one which has been abraded or broken, by fitting a crown thereto. Although the principles of the invention are disclosed with reference to a gold crown, it finds equal utility with crowns of other metallic media as well as those of porcelain and other non-metallic media.
Cases requiring endodontia (root canal therapy) are, in nearly every case, coronally weak to begin with. During the root canal treatment, they are further weakened by the need to gain adequate access to the canal or canals. In turn, this requires reinforcement of the remainder of the tooth before placement of the crown. This is usually done by the use of a post fitting within the canal and a hard core integrated with the post to receive the crown. The post usually occupies the upper two-thirds of the canal and the core is usually located in the pulp chamber as well as in a portion of the stub of the tooth. This core need be built only high enough to mount and reinforce the final crown. The post and core assembly is usually of gold and is cemented in place.
One direct method, heretofore known, is referred to as the "Adaptic" technique and uses a metal post cemented into the canal, pins screwed into the stub and a core built up on the post and pins by employing a plastic composition, e.g. that known as Adaptic. This composition is a restorative material available from Johnson & Johnson, 20 Lake Drive, East Windsor, N.J. 08520. Adaptic is the trademark under which the product is sold. The mentioned post and pins are secured in the stub and the pins are provided with enlargements or hooks at the outer end adapted to interlock with a mass of air-hardenable plastic retained in position against the stub by means of a "crown form" pending hardening. However, following hardening and removal of the crown form the operator must grind off a large volume of plastic to produce a core of the desired size and shape. This last is, in general, the method pursued when preparing a tooth for a porcelain or gold crown. However, the patient is discommoded and inordinate time consumed.
One method somewhat favored for front teeth is to fill a transparent crown form with a plastic moldable material, position it over the post which has been previously secured in the canal and allow the material to set.
In one indirect method, a metal or plastic post is located in a die and the core is built up outside the mouth with wax, drop by drop, to the approximate size and is then shaped with an instrument. The post and core is separated from the die and the casting is made by the lost-wax technique.
Another indirect method entails a rubber impression of the stub of the tooth including the hole in the canal and a die or model is then made from this impression. Then the post (metallic or plastic) is located in the model and the wax core portion is made on it. Casting is by the classical lost wax process. Should the dentist desire to vary this method by indirect steps, considerable time will be saved since, instead of building up the core (in this case, wax) drop by drop, he can still use a core form as he would in the mouth.
In any of the indirect techniques, fabrication of the core outside of the mouth is extremely time-consuming and, hence, uncomfortable for the patient.
When using wax for the core portion one can employ either a post of metal, e.g. gold, or plastic but neither is cemented in the root canal. On the contrary the post is removed after the wax core has been built thereon in order that the casting may be made in a crucible. This casting will include a replica of the post which is then cemented into the tooth. When using a metal post, the wax is replaced by the poured metal. When using a plastic post, the core portion, together with the post portion, are cast in one piece.